Astronomy Made Simple [Revised] 0385020899, 9780385020893

Citation preview

Astronomy Made Simple

Only $1.95 A comprehensive course for self-study and review Revised edition By Meir H.Degani

Digitized by the Internet Archive in 2018 with funding from Kahle/Austin Foundation

https://archive.org/details/madesimpleselfteOOOOdega

ASTRONOMY MADE SIMPLE REVISED EDITION

BY

MEIR H. DEGANI, Sc.D. Chairman, Science Dept., State University of N.Y., Maritime College

MADE SIMPLE BOOKS DOUBLEDAY & COMPANY, INC. GARDEN CITY, NEW YORK

Copyright ©

1955, 1963

by Doubleday & Company, Inc.

ALL RIGHTS RESERVED PRINTED IN THE UNITED STATES OF AMERICA

TABLE OF CONTENTS A BRIEF HISTORY OF ASTRONOMY

.

7

CHAPTER I THE UNIVERSE.8 PART 1: BUILDING BLOCKS

Why Study Astronomy?.8 The Building Blocks.8 Planets .«»»....«. 0 Planetoids.9 Satellites.9 Comets •••••••••• 9 Meteoroids.10 Stars.10 Nebulae.10 PART

2:

ORGANIZATION

The Sun •••••••••• 11 Planets.fl The Solar System.12 The Stars.12 Our Galaxy.12 Other Galaxies.12 The Universe.12 Outline of the Universe.. 13

CHAPTER II

How to Use the Map Apparent Daily and Annual Motion of the Stars .

PART

3:

•

...

13

PART 1: THE BIG DIPPER The Stars of the Big Dipper ..... 14 Scale of Angular Distances . . 14 Legends.14 Other Names.15 Apparent Brightness of Stars.15 Hipparchus’ Classification of Stars according to Brightness.15 Decimal Division of Apparent Magnitudes . . 15 Relation between Apparent Magnitude and Ap¬ parent Brightness.15 Zero and Negative Values of Apparent Magnitude 16 Determining Apparent Magnitudes .... 16 Apparent Daily Motions of Stars .... 17 The Apparent Annual Motion of the Stars . . 18 PART 2: CIRCUMPOLAR STARS Latitude and North Circumpolar Stars ... 19 North Circumpolar Constellations .... 19 The Little Dipper.19 The Stars in the Little Dipper .... 20 Legends.20 The North Star in Navigation.20 The Celestial Poles.21 The Celestial Sphere.21 The Celestial Equator.21 Cassiopeia and Cepheus.22 Tycho’s Star.22 Cepheids.22 The Milky Way.22 Legends About Cassiopeia and Cepheus ... 23 The Constellation Dragon.23 The Star Thuban, or Alpha-Draconis ... 23 A Map of North Circumpolar Constellations . 23 Names of Stars.23

9

9

•

February Constellations The Stars in Orion Locating Orion in the Sky Orion in Legend . The Stars in Auriga Locating Auriga . March Constellations The Stars in Gemini The Stars in Canis Minor The Stars in Canis Major Legends about the Dogs Locating the Twins and the Dogs April Constellations The Stars in Leo Locating Leo The Sea Serpent May Constellations June Constellations The Stars in Bootes Locating Bootes The Stars in Virgo Locating Virgo July Constellations The Stars in Scorpius The Stars in Corona Borealis Corona Borealis in Legend August Constellations The Stars in Hercules September Constellations The Stars in Lyra Legends about the Lyre The Stars in Cygnus The Stars in Sagittarius • The Stars in Aquila October Constellations The Stars in Piscis Austrinus November Constellations The Stars in Pegasus Locating Pegasus December Constellations The Stars in Andromeda Andromeda in Legend • January Constellations • The Stars in Perseus The Stars in Taurus • The Stars in Eridanus .... •

•

9

9

9

•

•

9

9

9

•

•

9

9

9

•

•

9

9

9

•

•

9

9

9

•

•

9

9

9

.

•

•

9

9

9

.

•

•

9

9

9

.

•

•

9

9

9

.

•

•

9

9

9

•

•

9

9

9

•

9

9

9

•

•

9

9

9

•

•

9

9

9

•

•

9

9

9

•

•

9

9

9

•

•

9

9

9

•

•

9

9

9

•

•

9

9

9

•

9

9

9

9

•

9

9

9

9

•

9

9

9

9

•

9

9

9

9

•

9

9

9

9

•

9

9

9

9

.

•

•

9

9

9

.

•

•

9

9

9

•

•

9

9

9

•

9

9

9

9

•

9

9

9

9

•

O

9

9

9

9

9

9

9

•

9

9

9

9

•

9

9

9

9

•

9

9

9

9

•

9

9

9

9

•

9

9

9

9

•

9

9

9

9

•

9

9

9

9

•

9

9

9

9

9

9

9

9

9

9

9

9

9

9

9

9

9

9

9

9

9

9

9

.

.

....

.

.

.

.

.

.

.

.

.

•

PART

4:

24 24

NON-CIRCUMPOLAR STARS

.

STARS WITHOUT A TELESCOPE

•

SOUTH

The Stars in Carina The Stars in Crux The Stars in Centaurus

CIRCUMPOLAR

.

25 26 26 27 27 27 27 27 28 28 29 29 29 29 30 30 30 30 31 31 31 31 32 32 32 32 33 33 34 34 35 35 35 35 36 36 37 37 37 37 37 38 38 38 39 39

STARS

•

9

9

9

9

•

9

9

9

9

•

9

9

9

9

40 40 41

CHAPTER III ELEMENTS OF NAVIGATION.41 Parallels of Latitude.42 Meridians.42 The Celestial Sphere.43 Parallels of Declination.43 Hour Circles.44 Effect of Latitude on View of the Sky ... 45

CHAPTER VII

CHAPTER IV VIEWING STARS WITH A SMALL TELESCOPE

46

PHYSICS OF THE STARS.76

Selected List of Observations.47 Through the Telescope: North Circumpolar Constellations

...

47

February Observations.49

PART 1: TEMPERATURE OF STARS

Computing Stellar Temperatures

Other Methods of Determining Temperatures

March Observations.49 April Observations

.......

50

June Observations.51 July Observations.51 August Observations ...... 52 September Observations.

52

November Observations.54 December Observations.54

....

PART

2:

76

.

77

STELLAR DISTANCES

Direct Method.77 Units of Stellar Distances.79 Indirect Method for Measuring Stellar Distances . 80 Cepheids.80 Absolute Magnitude.80 Period of Cepheids and Absolute Magnitude

.

81

Determination of Distance with Absolute Magnitude

81

January Observations.54 South Circumpolar Observations ....

55

CHAPTER V

PART

.

56

SIZE OF STARS

Size of Stars as Determined by the Interferometer

82

Size of Stars as Determined from Luminosity

83

PART

TELESCOPES AND THEIR USES .

3:

4:

STELLAR MASSES AND DENSITIES

Mass of Visual Binaries.84 Mass of Stars of High Surface Gravity

PART 1: THE REFRACTING TELESCOPE

.

.

.

.

84

Densities.85 .....

56

Tracing of Light Rays.

57

A Simple Refracting Telescope ....

58

Radial Velocity.85

Chromatic Aberration (Color Defect of Lens)

58

Tangential Velocity.86

Spherical Aberration (Shape Defect of Lens)

60

Proper Motion.86

Corrected Objectives.

60

Space Velocity of Stars.87

Eyepieces.

60

The Huygenian Eyepiece

61

The Process of “Seeing”

.

PART

5:

STELLAR MOTIONS

PART 6: STELLAR SPECTRA

The Kellner Eyepiece.

61

Spectral Classes.87

The Orthoscopic Eyepiece.

61

Spectral Classes and Surface Temperatures of Stars

Coating of Lenses..

62 63

The Russell Diagram.88 The Normal Stars.89

63

The Abnormal Stars.90

The Three Powers of the Telescope The Light Gathering Power of a Telescope

.

63

The Resolving Power of a Telescope The Magnifying Power of a Telescope

PART

64

.

66

Telescopic Mountings. PART 2: THE REFLECTING TELESCOPE

Tracing of Light Rays .......

67

7:

88

RADIO ASTRONOMY

CHAPTER VIII THE NATURE OF STARS AND INTERSTEL¬ LAR SPACE.90

The Silvering of the Mirror.68

1:

PART

“PECULIAR” STARS

Layout of Optical Parts.68 The White Dwarfs.91

Similarities Between Refracting and Reflecting Types of Telescopes.69 PART

3:

The Red Giants

........

91

The Cepheids

........

91

RR-Lyrae Variables

GREAT TELESCOPES AND THEIR USES

.

92

Long Period Variables. The 200" Telescope.69 The Greatness of the 200" Telescope . . . 70 The Program of Research.70 Early Accomplishments.71 Photography

.........

PART

4:

71

92

Novae.92 Supernovae.

93

PART 2: THE SPACE BETWEEN THE STARS

Interstellar Gas.94 Interstellar Dust.94

THE RADIO TELESCOPE

Nebulae

.

95 PART 3: STELLAR ENERGY

CHAPTER VI

The Gravitational Theory.96

THE CHEMISTRY OF STARS: SPECTROSCOPY

72

The Spectroscope.72 The Prism Spectroscope

Energy from Nuclear Sources: Main Sequence Stars

96

Energy from Nuclear Sources: Red Giant Stars

.

97

The Production of Energy by White Dwarfs .

.

97

......

72

The Grating Spectroscope ......

73

Comparison of Spectroscopes

.....

73

The Birth.98

........

74

The Infancy.98

75

The Three Stages of Youth.98

Kinds of Spectra

Star Spectrum.

.

PART

4:

LIFE HISTORY OF A STAR

Adulthood.

99

The Later Years.

99

The Death.

99

CHAPTER XI THE INNER PLANETS.122 PART 1: THE PLANET MERCURY

CHAPTER IX

Basic Data.122

THE GALAXIES.

100

Times for Observations.122 Transits.122

PART 1:

OUR GALAXY

Phases.123 Studies of the Galaxy with the Aid of Star Counts

101

Studies of the Galaxy with the Aid of Globular Clusters

.

Rotation of the Galaxy PART 2:

....

Librations.123

•

•

101

•

•

102

Temperature.124 Albedo.124 Surface Gravity and Velocity of Escape .

OTHER GALAXIES

The Large Magellanic Cloud

.

•

102

The Small Magellanic Cloud

.

•

103

The Local Group of Galaxies

.

•

103

....

•

103

Remote Galaxies.

•

104

Classification of Galaxies

....

•

104

The “Red Shift”.

•

104

The Expanding Universe ....

•

104

The Andromeda Galaxy

A Day Equals a Year.123

.

.

125

PART 2: THE PLANET VENUS Basic Data.125 Telescopic Observations.126 Phases and Apparent Diameter

.

.

.

.

126

Brightness.126 Transits.126 The Clouds.127 The Atmosphere.127 Surface Temperature.127

PART 3: THE BIRTH OF THE UNIVERSE

The Preliminary Stage.

105

The Expansion.

105

The Formation of the Galaxies

•

105

The Size of the Universe

....

•

105

Looking into the Stars.

•

105

Greenhouse Effect.127 Rotation.128 Life on Venus.128 The Trip to Venus.128

CHAPTER XII CHAPTER X

THE EARTH AND ITS MOON

THE SOLAR SYSTEM.

•

•

106

PART 1: THE SUN

....

129

PART 1: THE EARTH Basic Data.129

Solar Data.

•

106

Rotation.129

Distance.

•

107

The Shape of the Earth.130

Diameter.

•

107

Revolution about Sun.

131

Volume.

•

107

Inclination of Equator to Ecliptic ....

131

Mass.

•

107

Precession of Equinoxes.132

Density.

•

108

Nutation.134

Surface Gravity.

•

108

The Interior of the Earth.134

The Photosphere. Sunspots.

•

108

Magnetism of the Earth.135

•

108

The Atmosphere.135

Rotation of the Sun.

•

110

The Atmosphere in Astronomy

Inclination of the Sun’s Axis

•

111

.

.

.

.

.

136

PART 2: THE MOON

The Effect of Sunspots on the Earth’s Magnetic Field.

111

The Effect of Sunspots on Auroras .

111

Basic Data.. . 137 The Moon in Its Orbit.137

The Effect of Sunspots on Terrestrial Affairs The Reversing Layer. •

112

Ocean Tides on Earth.138

112

Phases of the Moon.139

The Chromosphere. Prominences.

•

113

Two Kinds of Month.140

•

113

The Moon’s Path about the Sun

Chromospheric Flares.

•

113

The Moon’s Rotation about Its Axis

The Corona.

•

114

Librations.142

The Spectroheliograph

•

115

The Moon’s Surface.143

•

....

.... .

.

.

140 141

Surface Gravity of the Moon.144 PART 2:

THE

MECHANICS OF

THE SOLAR SYSTEM

Kepler’s First Law of Planetary Motion

•

•

Temperature of the Moon.144 116

Life on the Moon.145 A Trip to the Moon.145

Kepler’s Second Law of Planetary Motion

•

•

117

Kepler’s Third Law of Planetary Motion

•

•

118

Evaluation of Kepler’s Three Laws .

•

•

118

Newton’s Universal Law of Gravitation .

•

•

118

# • Apparent Motion of Planets as Seen from the Earth.

119 119

Duration of a Lunar Eclipse.148

Sidereal and Synodic Period of a Planet

120

Partial Lunar Eclipses.148

Application of the Law of Gravitation .

•

•

CHAPTER XIII ECLIPSES OF THE SUN AND THE MOON Conditions for a Lunar Eclipse

.

145

....

147

•

•

148

The Spinning of the Planet about Its Axis .

163

Eclipses of the Sun

•

•

148

The Satellites.

164

Series of Solar Eclipses

•

•

149

Through the Telescope.

164

Catalog of Eclipses

•

•

150

A Description of a Total Solar Eclipse

•

150

Scientific Interest in Solar Eclipses

9

151

Series of Lunar Eclipses

.

•

CHAPTER XIV THE OUTER PLANETS PART

.

•

©

©

•

The Orbits of Mars and the Earth

•

•

152

The Surface of the Planet

•

•

The Atmosphere of Mars .

•

•

152 153

Composition

e

•

154

.

•

•

154

....

•

•

155

•

•

155

.....

Seasons and Climates on Mars Recent Investigations

.

Satellites of Mars

....

•

•

155

A Trip to Mars

....

•

•

155

PART

2:

©

156

Theoretical “Discovery”

•

•

•

•

156

Orbits of the Planetoids

e

•

•

•

157

The Periods of the Planetoids

.

•

•

•

•

157

Our Interest in the Planetoids

.

•

•

e

•

157

.

....

Rotation of Jupiter about Its Axis The Satellites

©

9

•

158

•

•

•

158

•

•

•

159

....

Through a Telescope

.

.

....

Rotation of Saturn about Its Axis Rings

©

©

•

.

PART 1: THE COMETS

The Structure of a Comet.

169

The Orbit of Comets.

170

Life and Death of a Comet.

171

.

How to Identify Meteorites.

CHAPTER XVI 174

CHAPTER XVII THE ORIGIN OF THE SOLAR SYSTEM .

176

The Double Star Encounter Hypothesis The Turbulence Hypothesis.

178

•

©

•

•

•

161 161

•

172 174

177 178

•

•

Frequency

The Tidal and the Collision Hypotheses

160 160

9

168

APPENDIX HOME-BUILT TELESCOPES.

180

162

Home-Made

Refractor.

180

162

Home-Made Reflector. Mirror Grinding.

183 183

THE PLANET URANUS

Basic Data

.

The Discovery

COMETS AND METEOROIDS.

177

•

.

5:

CHAPTER XV

160

«

The Satellites

PART

PART 8: SOLAR SATELLITES

The Centrifugal Force Hypothesis .

•

......

Through a Telescope

166

159

THE PLANET SATURN

Basic Data

..

ARTIFICIAL EARTH SATELLITES

THE PLANET JUPITER

Basic Data

The Atmosphere

166

PART 2: METEOROIDS (METEORITES)

.

4:

Through the Telescope.

PLANETOIDS

Basic Data

PART

The Satellites.

165 165

Basic Data 152

The Atmosphere

The Discovery.

THE PLANET MARS

•

3:

165

PART 7: THE PLANET PLUTO

1:

•

PART

Basic Data.

152

Basic Data.

“Life” on Mars

PART 6: THE PLANET NEPTUNE

....

•

9

•

6)

163 163

GLOSSARY INDEX

.........

187 191

A BRIEF HISTORY OF ASTRONOMY The history of Astronomy may be con¬ veniently divided into three periods: the Geocentric, the Galactic, and the Universal. The first had its beginnings in ancient history, and came to a close in the 16th century. The second extends from the 17th through the 19th centuries. And the third began and con¬ tinues in the present century.

THE GEOCENTRIC PERIOD Early astronomers believed the earth to be in the center of the universe; and assumed that the sun, moon, and stars revolved about that stationary earth. Their interest, hardly scien¬ tific in our sense of the term, was mainly in practical matters, in the real and supposed relation of celestial events to those on the earth; in searching the skies for clues to good and evil omens. Even so, remarkable discoveries were made then. The calendar was developed with great accuracy. The apparent path of the sun among the stars—the ecliptic—was carefully defined. The complete cycle of solar and lunar eclipses was determined. And as early as the second century b.c., the motion of the earth’s axis was well understood. The great figure of Nicolaus Copernicus (1473-1543) is closely associated with the end of the primitive Geocentric period in the 16th century.

ingly scientific, motivated largely by the desire to know, to understand the basic laws govern¬ ing the motion of heavenly bodies, to explain what the eye saw. Progress from the 16th through the 19th centuries resulted from the effective combina¬ tion of extended observation, improved in¬ struments, and the work of scientific genius. Observation. Great quantities of data of fundamental importance were painstakingly gathered by careful observers, chief among whom is the great name of Tycho Brahe (1546-1601). Instruments. The introduction of the tele¬ scope in 1610 by Galileo Galilei was, of course, a milestone in the development of the science of Astronomy; as was the later invention and introduction of the spectroscope. The two in¬ struments complement one another: the tele¬ scope permits us to see the stars more clearly; the spectroscope analyzes stellar light, furnish¬ ing us with much information about the stars. Genius. Like every science. Astronomy re¬ quires for its advancement the labors of great minds that are able to apply to the observed data insight, imagination, intuition, as well as great learning. Such minds were Johannes Kepler (1571-1630) and Sir Isaac Newton (1642-1727): Kepler by the discovery of the laws of planetary motion, and Newton by the discovery of the Universal Law of Gravitation.

THE GALACTIC PERIOD

THE UNIVERSAL PERIOD

Modern Astronomy can be said to have begun in this period. Copernicus demonstrated that the earth, far from being the center of the universe, was merely one of the planets re¬ volving about the central sun. Hardly unique, the earth was found to be a quite ordinary planet, going through ordinary motions in an ordinary way. Indeed the central sun itself was realized to be merely one star among the multitudes of the heavens, one among a vast galaxy of simi¬ lar stars in every direction about us—some larger, some smaller, some heavier, some lighter than our sun. In this period the approach became increas¬

Now it became apparent that the galaxy of stars to which our sun belongs is merely one of many galaxies—some larger, some smaller than ours. To these much of the astronomical research of the last half century has been devoted, in an effort to achieve a “complete” picture of the universe. For this attempt ever greater telescopes have been constructed. The great theoretical genius associated most closely with this period in the public mind (although he was primarily a physicist and mathematician) is the late Dr. Albert Einstein (1879-1955). This is the astronomic period in which we live. And it is far from concluded.

CHAPTER I

THE UNIVERSE PART 1: BUILDING BLOCKS

ever man has looked up, there was the sky— always confronting him with seemingly im¬ ponderable problems, always challenging him to solve its mysteries. On one level, man has stated his reaction in magic and mythology, and this is permanently embedded in the world’s art, literature, and religions. On another level, he has attempted to explain the celestial phenomena perceived by his senses in scientific terms—and those explanations are the subject matter of the science of Astronomy.

INTRODUCTION AND DEFINITION For as long as man has been conscious of himself and the universe he inhabits, he has regarded the sky with awe and wonder—a source of constant and compelling fascination. Awe and wonder generate study and science; and from its beginnings in primitive, childlike stargazing, as man sought ceaselessly to con¬ quer ignorance and solve mysteries, there emerged finally the science of Astronomy. Astronomy is the science of the position, motion, constitution, history, and destiny of celestial bodies. In the course of its develop¬ ment as a science, it has already discovered many of the basic laws governing those bodies. But it is the nature of scientific investigation that its work is never done—and here, as else¬ where, immense labors remain to be per¬ formed.

THE BUILDING BLOCKS The “earth” we live on is a planet—one of a number of planets that revolve about the sun. The unassisted eye is capable of detecting several planets, one satellite (our moon), several thousands of stars; and, several times during the year, showers of shooting stars become clearly visible. Stars, planets, and satellites are some of the “building blocks” of the universe. The uni¬ verse is composed of these “blocks” in much the same way that a community is composed of homes, churches, hospitals. Among the distinct kinds of celestial (heav¬ enly) bodies are planets, planetoids, satellites, comets, meteors, stars, and nebulae. These are the building blocks of the universe; the known physical universe Is an immense assembly of these blocks. To the best of our knowledge, the universe consists, primarily, of nine planets, thirty-one satellites, a vast number (billions) of stars and other classes of celestial bodies.

WHY STUDY ASTRONOMY? Each in his own way and for his own pur¬ poses, professional astronomer and amateur “‘stargazer” alike, studies the science because it yields essential information about the uni¬ verse; and that information then becomes available for numerous practical and scientific applications. But to ask this question is like asking: “Why learn to read?” For the amateur espe¬ cially, there is more even than the knowledge to be gained, or the gratification of intellectual curiosity. We study Astronomy because the intelligent, inquiring mind must ask questions and seek answers; must know “Why?” and dis¬ cover “How?” And from the beginning, when¬

8

Astronomy Made Simple

PLANETS Planets are large, solid, nearly spherical masses, revolving about the sun in nearly circular orbits. The best known to us is, of course, our own earth. All of them are rela¬ tively cool and are made visible by reflected sunlight; several can be seen at one time or another by the unaided eye. Three planets, however, can be seen only with the aid of a telescope. At first glance, planets look very much like the multitude of stars that glitter in the sky; but an observer can identify a planet as possessing one or more of the following characteristics: A. Planets shine with a steady light, while stars do not. The light reaching our eyes from stars seems to change rapidly in both color and brightness. These changes in color and brightness cause the twinkling of the stars. B. Planets wander in the heavens: A planet which at one time was close to one star may later be observed close to another star. Stars, on the other hand, seem to keep the same positions relative to one another. See Fig. 1. The very word “planet” is derived from a Greek word meaning “wanderer.” C. Planets, when observed through telescopes, appear as small disks of light. The greater the magnification, the larger will be the diameter of the disk. Stars, even with the largest telescope, appear only as points of light. Even in the 200-inch telescope, they appear as mere points, having no meas¬ urable diameter. D. Planets may be found only in a narrow strip in the sky. Their motions are limited to the boundaries of this strip. Stars, of course, may be found in any part of the sky.

PLANETOIDS Planetoids are small, irregularly shaped solid bodies revolving, like the major planets, about the sun, and differing from planets primarily in size. They are also known either as Asteroids or as Minor Planets. The largest planetoid, Ceres, has a diameter of 480 miles; but many of them have a diameter of only two miles. The first planetoid was discovered on

9

January 1, 1801; many more have since been discovered. It is estimated that nearly 50,000 planetoids can be photographed with one of the large telescopes. They, too, shine by reflected sunlight; how¬ ever, because of their small surface, the amount of reflected light is very small. They cannot be seen without the aid of a telescope.

SATELLITES Six of the nine major planets have one or more moons revolving round them. These are called satellites. The earth has only one moon (satellite), while the planet Jupiter, for ex¬ ample, has twelve. To date, thirty-one satel¬ lites have been discovered, the last as recently as 1951.

COMETS Comets are celestial bodies of unique form and large size which appear from time to time. A typical comet consists of a luminous sphere, or head, connected to a long, tenuous cylinder, or tail. The head may seem as large as the sun; the tail describes an arc in the sky. To the naked-eye observer a comet appears as motionless as the moon. Actually it moves at speeds of hundreds of miles per second. The exact speed can be determined from its chang¬ ing position relative to the fixed stars. There are approximately a thousand known comets, and several new ones are discovered every year. The vast majority are too faint to be visible to the naked eye. Fairly great comets are rather rare; these appear, on the average, once or twice in a lifetime. The most recent of these spectacular objects appeared in December of 1947. Of the thousand or so known comets, nearly a hundred are known to move in “closed orbits”—that is, in elongated and cigar¬ shaped paths. The fact that the orbit is “closed,” has no beginning or end, is of great importance. Comets moving in them go round the same path continuously; many of them have been observed several times during their returns to the vicinity of the earth. The orbits of the other 900 comets have not been definitely ascertained. It is believed that

Astronomy Made Simple

10

O MARS

\

1 \

\ \

\ \* \

1 \

\

\ \

O

\ \

\

\

\

WARS

\ /

\ MARCH 10

mS

MARCH 20 I95S

MARCH 30 1955

1. Views of the same part of the sky on three different dates, March 10, March 20, and March 30, 1955. Note that the stars maintain the same relative position. The planet (Mars) has wandered considerably in that time.

Fig.

most of these also move in closed orbits and hence will be seen time and again when they come close to the earth, while the rest very likely make only one appearance in the vicin¬ ity of the earth, coming, probably, from outer space, making a U-turn, and then leaving, never to be seen again.

METEOROIDS* Meteoroids are usually tiny (about the size of the head of a pin), solid objects traversing through space. Occasionally a group of me¬ teoroids is attracted to the earth and becomes entangled in its atmosphere. The heat result¬ ing from this encounter consumes the object; the dust resulting from this cremation falls to the earth. Hundreds of tons of meteoric dust descend each year. On rare occasions large meteoroids manage to reach the earth before they are consumed. The light phenomenon which results from the entry of the meteoroid into the earth’s atmosphere is called meteor, or “shooting star,” the glow of which may persist several seconds.

STARS Stars are large globes of intensely heated gas, shining by their own light. At their sur¬ face, they reach temperatures of thousands of degrees; in their interior, temperatures are much higher. * The revised terminology for meteoric phenomena is used here. This revision was approved by the International Astronomical Union in 1961.

At these temperatures, matter cannot exist either in solid or in liquid form. The gases constituting the stars are much thicker than those on the earth usually are. The extremely high values of their density are due to enor¬ mous pressures which prevail in their interior. Stars move about in space, although their motion is not immediately perceptible. No change in their relative position can be de¬ tected in a year. Even in a thousand years, the stars will seem not to have moved substan¬ tially. Their pattern now is almost exactly that of a thousand years ago. This seeming fixed¬ ness is due to the vast distance separating us from them. At these distances it will take many thousands of years for the stellar pattern to undergo a noticeable change: This apparent constancy of position accounts for the popular name “fixed stars.”

NEBULAE A nebula is a vast cloud composed of dust and gas. The gases which compose it are ex¬ tremely thin and of low temperature. Nebulae do not shine by their own light, but are made visible by the light of neighboring stars. (These are known as Bright Nebulae.) When they are so visible, they appear to the unaided eye not unlike a fuzzy star. Their actual size and structure, however, can be determined only with the aid of a telescope. Other nebulae are dark, obscure the stars beyond them, and are not visible to the unassisted eye.

Astronomy Made Simple

11

PART 2: ORGANIZATION

INTRODUCTION Thus, the universe is composed of stars, nebulae, planets, comets, and other celestial bodies. Here, the blocks are assembled to form the plan of the universe. The planets, planetoids, satellites, comets, and meteorites revolve about a single star: the star we call the sun. Together they form the Solar System. The sun, and billions of other stars, form the community of stars known either as the Galaxy, Our Galaxy, or the Milky Way Galaxy. The universe contains many such stellar communities, or galaxies. Stellar distance is of an order of magnitude entirely different from that of planetary dis¬ tance: the former is enormously greater than the latter.

Fig. 3. Other objects, too, appear smaller with increas¬

ing distance. Note the apparent size of the distanrtree.

Fig. 4. The oval curve suggests the circumference of

the whole universe. The dot represents the location of the sun.

NEAR SUN

and light—energy of fundamental importance in maintaining life. The oval curve in Figure 4 represents the universe and the dot the posi¬ tion of the sun within the universe. (Note that Fig. 4, as well as Figs. 5, 6 and 7, are symbolic representations and not figures drawn to scale.)

Fig. 2. The sun is just an ordinary star. All the other

stars look tiny, as they are so remote that we see them only as mere points of light.

Distances between galaxies are still greater than distances between stars. In attempting to visualize unimaginable distances it is essential to use a scale. The plan of the universe on such a scale is given later in this section. THE SUN Although it may not seem so, the sun is just an ordinary star, similar to numerous other stars that we see in the sky. The sun appears large to us because it is, relatively speaking, near to us. All other stars appear as small points of light in the sky be¬ cause they are far away. See Fig. 2. Our in¬ terest in this star (the sun) derives from the fact that the earth receives from it both heat

PLANETS There are nine planets revolving about the sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Mercury is closest to the sun, and at a somewhat greater distance is Venus; then, the earth; and the farthest known planet from the sun is Pluto. The earth is 93 million miles from the sun. This distance is often referred to as an Astro¬ nomical Unit. Mercury is only four-tenths the earth’s distance from the sun. Pluto, the most distant planet, is forty times the earth’s dis¬ tance. The distance of Pluto can be stated as forty times ninety-three million miles, or simply as forty Astronomical Units. A reducing scale may help to visualize these distances. The scale that is commonly used represents the sun-earth distance as one foot long:

Astronomy Made Simple

12

93 million miles equal 1 foot; or,

be accommodated in a circular house of forty

1 Astronomical Unit equals 1 foot.

foot radius. The closest star, by our scale,

On this scale, Mercury is four-tenths of a

would be in a house fifty miles away. Other

foot; Venus is seven-tenths; and the earth is

stars, by our scale, are at scale distances of

one foot away from the sun. The farthest

thousands and hundreds of thousands of miles

planet is forty feet from the sun. A circular

from the sun.

box of forty foot radius could accommodate all the planets. The box could be quite shal¬ low, as all the planets move approximately in the same plane.

•

OUR GALAXY

These stars form a large community called Our Galaxy or the “Milky Way” Galaxy. It is estimated that the number of stars in Our Galaxy is close to a hundred billion—other¬

THE SOLAR SYSTEM The sun and the planets are the major com¬ ponents of the Solar System. Other members of this system are: 1. the host of smaller planets known as as¬ teroids or planetoids; 2. the several moons, known as satellites, that revolve about six of these planets; 3. comets that appear from time to time; 4. the vast number of meteoroids.

wise stated as 100 X 109, or a hundred thou¬ sand million. The outer surface of the Galaxy is often compared either to a grindstone or to a lens. A top view of the Galaxy would reveal its circular shape as well as the spiral design formed by the stars. A side view would suggest its similarity to a lens, namely, that it is thick in the center, and thins out toward the edges. Again using the one foot scale, the diameter of the circle would be close to a million miles,

The circle around the dot in Figure 5 repre¬ sents the entire Solar System.

while the maximum thickness is only about one sixth of the diameter. Our Galaxy is represented in Figure 6.

Fig. 5. The oval curve suggests the circumference of

the whole universe. The dot and the small circle rep¬ resent the sun and the solar system, respectively.

Fig. 6. The oval curve represents the circumference of

the universe. Our Galaxy is indicated inside the oval. The dot and the circle represent the sun and the solar

THE STARS Distances to stars are immensely greater than distances to planets. Even the star nearest

system, respectively.

our own sun is at a distance of 270,000 Astro¬

OTHER GALAXIES Ours is not the only galaxy in the universe:

nomical Units. Using the scale (one foot equals

many have been discovered in recent years,

one Astronomical Unit or ninety-three million

strikingly similar to our own. The scale dis¬

miles), the star closest to our sun would be at

tances between them are from ten to twenty

a distance of fifty miles.

million miles. A highly simplified picture of

The two units should be carefully noted.

the universe is shown in Figure 7.

Distances between planets are stated in feet, while those between stars are stated in miles. A mental picture might help to visualize this

THE UNIVERSE The universe, then, consists of many gal¬

distinction. The sun, and all the planets, could

axies, as galaxies consist of billions of stars.

Astronomy Made Simple

13

a beam of light in one second is 186,000 miles; hence: One light year 186,000 X 60 (SeC°ndsN) X 60 Vminute/ \ hour / X 365 K = 5,880,000,000,000 miles Fig. 7. The “complete” universe consists of many gal¬ axies. One, containing the sun, is known as Our Galaxy, the Galaxy, or the Milky Way Galaxy.

One of these, the sun, is the star around which our earth moves, as do the other planets of our Solar System.

OUTLINE OF THE UNIVERSE IN TERMS OF ACTUAL DISTANCE The distance to the sun is 93 million miles; the distance to our nearest star, Alpha Centauri, is 25,000,000,000,000 miles, or 25 million million miles. Distant stars are inconceivably more remote. The mile unit is of no use in dealing with the distances of stars and galaxies—instead, astronomers use the unit ‘Tight year”: one light year is the distance that a beam of light travels in one year. The distance covered by

or six million million miles, approximately. The star nearest the Solar System is 4.3 light years away. The diameter of Our Galaxy is about 100,000 light years; its maximum thick¬ ness is 15,000 light years. An average distance between galaxies would be approximately a million light years. The sun is only a minute fraction of a light year from the earth. The distance to the sun may be stated as 8 light minutes. Distances to heavenly bodies, when stated in terms of light, have an added meaning—for the sun, it implies that it takes a beam of light 8 minutes to reach the earth. So for the stars. A ray of light from Alpha Centauri reaches the earth 4% years after leaving the star. The most distant object seen by the unaided eye is the Andromeda Galaxy—a million and a half light years away. The light entering the observer’s eye has been en route for that time.

CHAPTER II

STARS WITHOUT A TELESCOPE PART 1: THE BIG DIPPER

INTRODUCTION Astronomy is one of the several sciences engaged in the study of nature. Much remains to be learned, and many important discoveries can still be made without the use of any equipment. The sky is the laboratory. The time is any fine, clear evening. The place is outdoors, preferably away from city lights, with an unobstructed view of the sky. Even casual observation reveals that the

stars seem to be assembled in groups. Such a group is called a constellation. Modern astron¬ omy recognizes eighty-eight stellar constella¬ tions, many of which are of interest only to the professional astronomer. Only twenty of the eighty-eight may be considered well known—among them Orion, the Big and Small Dippers, the Dragon, and the Big and Small Dogs. The group easiest to identify is probably

Astronomy Made Simple

14

the Big Dipper. As its name implies, the stars form the outline of a dipper. It is important to become familiar with that group of stars as it is with reference to it that the locations of other constellations are most readily deter¬ mined. The Big Dipper can be seen every clear evening in most of the northern hemisphere. This section deals primarily with the stars of that constellation.

stated more formally as: The angle subtended at our eye by the diameter of the full moon is 0.5 degree. See Fig. 9. Another angular distance often used is the one between Dubhe and Merak—close to five degrees.

THE STARS OF THE BIG DIPPER

Seven bright stars form the pattern of the Dipper. The four forming the ‘‘bowl” are known as Dubhe, Merak, Phecda, and Megrez, all Arabic names: Dubhe means “bear,” Merak “loin,” Phecda and Megrez, “thigh” and “the root of the bear’s tail,” respectively. The stars forming the “handle” of the Dipper are known as Alkaid, Mizar, and Alioth, also Arabic names, meaning “the chief,” and “the apron”; the precise meaning of the name “Alioth” is still disputed. Close to Mizar is the small star Alcor. The Arabs called these two stars “the Horse and the Rider.” The star Alcor was used by them in a test for good eyesight. See Fig. 8.

Fig. 10. The angle subtended by Dubhe and Merak at

the eye of an observer is close to 5°. Ten full moons could be placed side by side in that angle.

Ten moons could be placed side by side in the distance between these two stars. See Fig. 10. PROBLEM 1:

Estimate the angular distance between Dubhe and Megrez. Answer: 10°, approximately. PROBLEM 2:

Find the North Star, using the following data: (See Fig. 16). a. The North Star is on the extension of the line joining Dubhe and Merak.

PHECDA

b. It is 29° from Dubhe, and, of course, 34° MERAJO-'"'

\

from Merak.

\

\

\

\ t

ALKAID ALIOTH \

LEGENDS WIZARD

DUBHE

“ALCOR

Fig. 8. The Big Dipper.

One of the early names given to this con¬ stellation was the “Great Bear” and the Arabic names meaning “thigh,” “loin,” etc., describe parts of the bear. See Fig. 11.

SCALE OF ANGULAR DISTANCES

Locations of stars are stated in terms of angles. The angular distance, measured in degrees, is the angle subtended by these stars at the observer. Fig. 11. The Great Bear. Note the position of the Big

Dipper. 9. The angular distance of the full moon is about half a degree. Fig.

It is of importance to be able to gauge small angles in the sky. The diameter of the full moon is about half a degree, otherwise

The reason for this is not known, as an observer Can scarcely imagine the outline of a bear or any other animal in that constellation. An ancient legend held that the Bear repre¬ sented Callisto, a daughter of the King of Arcadia, beloved of Jupiter, who, in order to

Astronomy Made Simple protect her, changed her into a Bear and trans¬ ferred her to the skies. Another legend held that the Great Spirit purposely put the Great Bear in the sky to act as a “calendar” for earthly bears. During the half year when the Great Bear is low in the sky, all earthly bears stay in their dens and keep warm. When the Bear is high in the sky, bears leave their dens, for summer has begun.

OTHER NAMES The names Great Bear, Big Dipper, are still in common use. The scientific name for the constellation is the Latin translation of Great Bear—Ursa Major (wr' sa ma' jer). In England, the constellation is known as the “Plough,” or the Wain (for wagon). note:

At times the term Big Dipper is used to refer

to the seven bright stars, and the term Great Bear or Ursa Major to refer to all the stars in the constella¬ tion. Usually, however, these terms are used inter¬ changeably.

APPARENT BRIGHTNESS OF STARS The seven stars of the Big Dipper differ materially in apparent brightness. The bright¬ est star is Alioth; the faintest, Megrez. Technically this is stated in terms of ap¬ parent magnitude. Alioth has the smallest apparent magnitude (1.7); Megrez, the larg¬ est (3.4).

HIPPARCHUS' CLASSIFICATION OF STARS ACCORDING TO BRIGHTNESS The ancient Greek astronomers classified the visible stars according to their apparent brightness, into six classes. This basic classifi¬ cation, in the main, is still valid. To Hippar¬ chus, who lived on the island of Rhodes in the second century b.c., goes the credit for this classification. The twenty brightest stars known to him were arbitrarily designated as stars of the first magnitude; and the next fifty in order of apparent brightness were desig¬ nated as stars of the second magnitude; and so on. The designation of sixth magnitude was given to several hundred stars barely visible to the normal human eye. See Fig. 12. Thus a completely arbitrary classification of

Fig.

15

12. The relationship between brightness and

magnitude.

stars, according to their brightness, was ob¬ tained. These magnitudes are, however, only apparent magnitudes. Some stars are actually bright, but appear faint because of their great distance.

DECIMAL DIVISION OF APPARENT MAGNITUDES In the 19th century, the decimal division was introduced. In this classification, a star of magnitude 5.5 has an apparent brightness halfway between that of a star of magnitude 5.0 and that of a star of magnitude 6.0. Simi¬ larly, to state that the North Star (Polaris) has a magnitude of 2.1 signifies that its apparent brightness is only slightly less than the bright¬ ness of a star of magnitude 2.0. Increasingly, the decimal method of denoting magnitudes has been applied more extensively and made more precise.

RELATION BETWEEN APPARENT MAGNITUDE AND APPARENT BRIGHTNESS Applying results from modern psycho¬ physics and physics (based on the psycho¬ physical law formulated by Fechner in 1859. The law states that if a stimulus increases in a geometric progression the sensation resulting from it increases in an arithmetic progres¬ sion), it was determined that magnitude 2 stars are 2.5 (more precisely, 2.512) times brighter than magnitude 3 stars. Similarly, magnitude 3 stars are 2.512 times brighter than magnitude 4 stars, and so on. problem

3:

The star Dubhe in the constellation Ursa Major has an apparent magnitude of 2.0. An unknown star, X, has an apparent magnitude of 4.0. How much brighter is Dubhe than star X?

Astronomy Made Simple

16

Solution: A decrease in one order of magnitude corresponds to an increase in 2.5 times in apparent brightness. A decrease in two orders of magnitude is the same as increase in 2.5 X 2.5 — 6.25 times in apparent brightness.

The following table contains a list of stars of known apparent magnitude. These can be used for the determination of magnitude of many other stars.

Answer: To the eye, Dubhe will appear more than six times brighter than the star X.

ZERO AND NEGATIVE VALUES OF APPARENT MAGNITUDE The twenty stars originally designated as first magnitude stars were subsequently re¬ grouped. This was necessary because some of the stars were much brighter than others. The brighter stars of this group were designated as having magnitudes of 0.9, 0.8, 0.7, etc., through 0.0 to negative numbers. The star with the greatest apparent brightness at night is Sirius. Its apparent magnitude is —1.6. On the same scale, the apparent magnitude of our sun is immensely greater: —26.7. DETERMINING APPARENT MAGNITUDES The method of determining the magnitude of stars by observation is rather simple. With practice, fairly accurate results (an accuracy of .1 of a magnitude) can be obtained. The method was used extensively by the German astronomer Argelander (1799-1875) and his associates in the preparation of the great star catalog, the “B.D. Catalog.” (B.D. is the ab¬ breviation of the German title of the catalog, “Bonner Durchmusterung”—“Bonn Cata¬ log.”) By this method, the observer compares the apparent brightness of a star with two or more neighboring stars of known magnitudes. Thus, a star that appears somewhat fainter than a neighboring star of 2.4 magnitude and somewhat brighter than another neighboring star of 2.6 magnitude, will be designated as having a magnitude of 2.5. In using this method it is advisable to make sure that: 1. The star to be measured and the known magnitude stars should be at about the same distance above the horizon. 2. The known magnitude stars should be as close as possible to the star to be measured. 3. One of the known magnitude stars should be somewhat brighter and the other some¬ what fainter than the star to be measured.

Constellation

Star

Apparent Magnitude

Alpheratz

Andromeda

2.2

Schedar

Cassiopeia

2.5

Diphda

Cetus

Achernar

Eridanus

2.2 0.6

Hamal

Aries

2.2

Acamar

Eridanus

3.1

Mirfak

Perseus

1.9

Aldebaran

Taurus

1.1

Rigel

Orion

0.3

Capella

Auriga

0.2

Bellatrix

Orion

1.7

Canopus

Carina

-0.9

Sirius

Canis Major

-1.6

Procyon

Canis Minor

0.5

Pollux

Gemini

1.2

Regulus

Leo

1.3

Dubhe

Ursa Major

2.0

Acrux

Crux

1.1

Arcturus

Bootes

Zubenelgenubi

Libra

0.2 2.9

Shaula

Scorpius

1.7

Nunki

Sagittarius

Markab

Pegasus

2.1 2.6

problem

4:

Determine which of the two is the brighter star, Alkaid or Merak.

Answer: Alkaid is the brighter one. The apparent magnitude of Alkaid is 1.9; that of Merak, 2.4. problem 5: Find three stars in the Big Dipper that appear to be of equal brightness.

Answer: Mizar, Merak and Phecda have almost the same apparent brightness. Precisely, they are designated as being 2.4, 2.4, and 2.5 magnitude stars, respectively. Phecda is by a very slight degree fainter than the other two. problem 6:

Determine the apparent magnitude of the North Star (Polaris).

Answer: Polaris is but slightly brighter than Merak, and slightly fainter than Dubhe. It is usu¬ ally designated as a 2.1 magnitude star.

Note again, this refers to apparent magni¬ tudes. Actually, Polaris is much brighter than our sun—in fact, nearly 1,500 times brighter. The great distance accounts for its being only a magnitude 2.1 star. Stated in terms of time, it takes light, traveling at the speed of 186,000 miles per second, 8% minutes to reach earth

17

Astronomy Made Simple

wwy\ •4 / /

310™H /

%v •4\

V' O'*

*,A

** Q

\ < \J \ \

V \ \

x >

2 O r m

«

X

POLARisV'

/\ / \

X

*

\

N

> \